Bluetooth: L2CAP: Fix l2cap_global_chan_by_psm

[ Upstream commit f937b758 ]

l2cap_global_chan_by_psm shall not return fixed channels as they are not
meant to be connected by (S)PSM.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Reviewed-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>